Antenna article
 

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote:
As built, both the pattern
and the impedance already seem to be what I want.

And it already has a counterpoise. The counterpoise is where the current
flowing on the inside of the coax shield goes when it leaves the coax. The
J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL
doesn't radiate (much) and is part of the matching section and counterpoise.
Quite often, the outside of the coax braid is also part of the counterpoise.
--
73, Cecil, w5dxp.com

Thanks, that's what I thought, too. The nature of current flow on the
outside of the coax is the reason so many construction articles mention
winding a few turns of the coax into a choke, not far* from the antenna. I
don't always do it but I probably should.

* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside? My head spins just a little.

"Sal"

Antenna article
 

On Sat, 15 Feb 2014 13:27:15 -0800, "Sal" salmonella@food
poisoning.org wrote:

I had a question about his j-pole analysis. He says "A J-pole, like ANY end
fed antenna, needs radials, a counterpoise or ground plane to work
properly."

I've never seen this anywhere else, even as a suggestion or hint. I can
envision places to fit one but what would it do? As built, both the pattern
and the impedance already seem to be what I want.

Thoughts?

I agree with the author (Terry Graves K7FE).

The rule-of-thumb is that an end fed antenna requires a shield ground
to act as a counterpoise and to keep the coax from radiating. The
J-pole (and Zepp) antennas are not exactly end fed, but are close
enough. A possible exception to the rule is a 1/2 wave end fed
antenna, which allegedly does not require a ground plane. I'm
undecided on that point.

However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane. This article covers the point (and more):
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
Quoting:
Summary End-feds Without Grounds
ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER
EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE
OR MAST COMMON MODE CURRENTS!
This is true for 5/8th waves, Zepp antennas, R7's, R5's, or
even common J-poles. End-feeding antennas is bad news unless
you have a large well-established ground at the feedpoint.
Even 1/4wl groundplanes have common mode problems. When I
designed a commercial 1/4 wave groundplane with four 1/4 wave
long radials, I had to insulate the radials from the mast and
isolate the coax shield from the mast and radials with a 1/4
wave stub that formed a choke balun. Without the decoupling,
I could change SWR simply by changing mast or feedline grounding.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Antenna article
 

On Sat, 15 Feb 2014 19:58:10 -0800, Jeff Liebermann
wrote:

http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

Not really a criticism, but more of an oddity. Starting on Pg 10, the
author shows that a J-Pole driving the LONG element has 2.37dBi gain,
while the same antenna driving the SHORT element has -3.17dBi gain.

I've never really looked as which way is the correct way to connect
the coax cable. I also don't have any J-Poles around the house.
Skimming the available photos:
https://www.google.com/search?tbm=isch&q=j-pole
I find a mix of methods. Most seem to do it the right way, but there
are plenty doing it wrong.
http://forums.radioreference.com/scanner-receiver-antennas/208290-j-pole-discrepancy.html
http://wmarc.wildmidwest.org/slide_shows/J-Pole_Antenna_Build/images/GEN_3764.jpg

This one is interesting because the two drawings show the correct
wiring, but the photograph shows it built backwards.
http://www.iw5edi.com/technical-articles/144-430-dual-band-jpole-antenna
Sigh.

I hate to admit it, but I think I've built them backwards over the
years. Maybe that's why J-poles have such a lousy reputation and why
I think they suck?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Antenna article
 

"Jeff Liebermann" wrote in message
...
On Sat, 15 Feb 2014 13:27:15 -0800, "Sal" salmonella@food
poisoning.org wrote:

I had a question about his j-pole analysis.

snip

I agree with the author (Terry Graves K7FE).

snip

However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane. This article covers the point (and more):
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
Quoting:
Summary End-feds Without Grounds
ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER
EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE
OR MAST COMMON MODE CURRENTS!

Hi, Jeff,

I can agree with the need for preventing feedline radiation but one thing
you and Terry say may be erroneous. I believe the radiating element of a
J-pole to be a half wavelength long, not a quarter-wave. I looked at
Terry's EZNEC wires list and observed the long side (the radiator or driven
element) of the J is 57 inches and the short side (the stub) is 19 inches.
(These dimensions agree with my idea of a 2m J-pole. I've made a few.)

As I understand the action of the J-pole, net radiation is low or nil from
currents in the lower third of the antenna (bottom third of the radiator and
the adjacent stub). The desired radiation comes from the top two-thirds (38
inches) of the radiator, which is very nearly a half wave at 2m. Allowing
for so-called "end effect," it's almost exactly cut to 146 MHz.

Please check my reasoning and math Your comments are welcome. Thanks.

73,
"Sal"
(KD6VKW)

Antenna article
 

On Sat, 15 Feb 2014 22:29:31 -0800, "Sal" salmonella@food
poisoning.org wrote:

I can agree with the need for preventing feedline radiation but one thing
you and Terry say may be erroneous. I believe the radiating element of a
J-pole to be a half wavelength long, not a quarter-wave.

You're correct. See my follow up posting. I've been building J-poles
driving the short 1/4 wave element instead of the long 1/2 wave
element.

I looked at
Terry's EZNEC wires list and observed the long side (the radiator or driven
element) of the J is 57 inches and the short side (the stub) is 19 inches.
(These dimensions agree with my idea of a 2m J-pole. I've made a few.)

As I understand the action of the J-pole, net radiation is low or nil from
currents in the lower third of the antenna (bottom third of the radiator and
the adjacent stub).

Correct. The original Zepp antenna was designed to be lowered from a
hydrogen filled Zeppelin dirigible. Any sparks or arcing caused by
high voltage from the antenna to the dirigible would be considered a
really bad idea. So, the antenna was designed to have zero voltage
and probably zero current at the closest point of the antenna to the
dirigible.

The desired radiation comes from the top two-thirds (38
inches) of the radiator, which is very nearly a half wave at 2m. Allowing
for so-called "end effect," it's almost exactly cut to 146 MHz.

Well, here we disagree slightly. I once made a similar comment in
this group about the radiation from an end fed collinear antenna came
mostly from the bottom of the antenna. I was corrected by Roy
Lewallen (W7EL) who pointed out that the current through the length of
such an antenna is the same and therefore the radiation from all parts
of the antenna are similarly identical. Unless I'm missing something
(a real possibility), the situation is the same with a J-pole.
https://groups.google.com/forum/?hl=en#!original/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ

This is interesting:
http://www.qikzepp.com/QikZepp_technical_information.html
It shows a 1909 German patent for a Zepp antenna. However, the
accompanying description once again makes the mistake of feeding the
1/4 WL section and not the longer 1/2 WL section:

Early fixed installation Zepp Antennas were a half-wavelength
long(or multiple) and fed with a 1/4 WL (or multiple) open
wire feed line which uses only one of the wires. The feed
line provided a matching section for the transmitter.

Please check my reasoning and math Your comments are welcome. Thanks.

I hate being wrong, but do I seem to be getting used to it.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Antenna article
 

On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:
However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane.

Sorry, that is not correct. Both a Zepp and a J-pole are 1/2WL antennas fed with a 1/4WL matching section of transmission line containing mostly balanced transmission line currents. Here is a plot of the two currents in the matching section. The difference in those two currents is the common-mode current which indeed does radiate. The point is that the average differential transmission line current is more than 10 times the average common-mode radiating current so the matching section is acting primarily as a transmission line, transferring RF energy to the primary radiating element which is indeed 1/2WL element.

http://w5dxp.com/ZeppCrnt.jpg

Does the matching section radiate? Of course it does. But because the common-mode current is a small percentage of the total current, it does not radiate much and transfers most of the RF energy to the 1/2WL radiating element.. And yes, that small percentage of common-mode current on the matching section indicates that it is also acting as the antenna counterpoise in the above graph. But most of the radiation from the antenna is from the center of the 1/2WL element, just as it is for a 1/2WL dipole. In fact, a Zepp meets the IEEE definition of a dipole because it contains two electrical poles.
--
73, Cecil, w5dxp.com

Antenna article
 

On Saturday, February 15, 2014 5:46:21 PM UTC-6, gareth wrote:
I wonder of perchance you are a professional?

I've been a ham for 60 years and worked as a EE for 38 years so you might say I am 2/3 professional.:)
--
73, Cecil, w5dxp.com

Antenna article
 

On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside?

Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer.
--
73, Cecil, w5dxp.com

Antenna article
 

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 5:46:21 PM UTC-6, gareth wrote:
I wonder of perchance you are a professional?

I've been a ham for 60 years and worked as a EE for 38 years so you might
say I am 2/3 professional.:)

44 and 42 for me, but mainly diverted into software engineering for 96% of
that time.

Antenna article
 

On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP
wrote:

On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:
However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven
element is a 1/4 wavelength long, and therefore DOES require a ground
plane.

Sorry, that is not correct.

Agreed. See my followup to my posting where I noticed that I've been
building J-pole antennas with the coax feed connected to the wrong
element. Judging by some of the photos I've found, I'm not alone.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558